Long-term Pattern Of Algal Responses To Catchment Development And Extreme Climate In Lakes Of Southeast Yunnan

Lakes in Yunnan-Guizhou Plateau provide important freshwater resources and vital ecosystem service for socio-economic development.However,with the sustained socio-economic development,many lakes and reservoirs have experienced severe threats such as deterioration of water quality,degradation of ecological and environmental functioning,and hydrological regulation,in the context of continued warming of regional climate and increased intensity and frequent of extreme climate.At the same time,the increased occurrence of extreme drought in Yunnan area during the past 60 years had led to the decreasing of water level.The demand of basic using water for residents had pushed this question to the edge.Here we presented well-resolved sediment records(including the proxies of grain size,element and stable isotope,pigments,diatoms)from four lakes(Yilong,Qilu,Chahei and Yuxian Lakes)of Southwest China,which had experienced significant water level changes over the past 60 years.The main conclusions are listed as follows.(1)Our multi-proxy lake sediment results showed that there existed clear differences in the trajectory of environmental change and ecological responses due to the strong difference in the type and intensity of catchment development over the past century.In the large and shallow lakes of Yilong and Qilu,nutrient enrichment and climate-hydrological factors were identified to drive the changes of both algal abundance and diatom assemblages over time.The records of geochemical and pigments proxies in Yilong revealed a trajectory of nutrient enrichment starting from the middle 1960 s,which led to the increase in algal abundance and the pronounced replacement of oligotrophic diatom taxa by eutrophic species since the 1970 s.Although eutrophication has been the paramount driver for the increase in algal abundance,diatom composition in Yilong Lake was found to be more affected by climate and hydrologic factors.Similarly,the nutrient and pigment records from Qilu Lake suggested that eutrophication and increased lake production started from the late1960 s and the early 1970 s.Consistently,diatom community shifted from an assemblage typically composed of oligotrophic taxa to one dominated by eutrophic taxa(e.g.,Cyclotella comta).The small lakes of Chahei and Yuxian showed aconcordant change around ca.2011,when the dominant diatoms shifted from limnetic species(i.e.F.construens and F.pinnata)to riverine taxa,such as Nitzschia spp.and Navicula spp.Over the past century,Chahei Lake was also influenced by increased nutrient input since the late 1970 s but this trajectory was not found in Yuxian Lake.(2)Our results also showed that the pattern of ecological changes varied between different biotic indicators in response to environmental changes in shallow lakes.The sediment concentration of pigments consistently responded to the increase of nutrient levels between lakes,while the shift of diatom community could capture the signals derived from eutrophication,hydrological change and extreme climate.Meanwhile,the same biological proxy(i.e.diatoms)also showed inconsistent patterns in response to the extreme drought events in large shallow lakes which differed in trophic levels and the magnitude of hydrologic changes.For example diatom succession was directly modulated by climate and hydrologic changes in Yilong Lake after ca.2005,when diatom community dominated by F.crotonensis,a planktonic species often found in nutrient-enriched waters,was replaced by small benthic Fragilaria.Comparably,the lakes of Chahei and Yuxian with generally lower nutrient levels showed a coherent response to the extreme drought event during the period of 2000-2013,with dominant diatoms shifting significantly from limnetic taxa to riverine taxa after ca.2011.While in Qilu Lake,the extreme drought and reduced lake levels showed an indirect effect on diatom assemblages through the concentrating of the nutrient contents,which led to the shift of dominant diatoms to the more eutrophic taxa such as F.crotonensis and C.pseudostelligera.